The method for purifying water with the use of a microorganism-attached packing material obtained by allowing microorganisms to attach on to the surface of a particulate packing material such as sand, anthracite, active carbon and a plastic filter medium to form a biomembrane is a so-called “biomembrane filtration method”, and various types of methods have been quite lately investigated. Among them, a method comprising sprinkling water to be treated over a packed layer in which microorganisms are attached on to a particulate packing material in a fixed bed state from above, and simultaneously flowing air or oxygen from the bottom of the packed layer to form a gas-liquid counter current state, thereby bringing the water to be treated into contact with air or oxygen to effect purification of the water to be treated, and intermittently washing the packed layer and discharging a slurry built up in the packed layer, has many advantages such that a slurry can be treated without circulation; bulking is not caused; clogging of the packed layer by the slurry is prevented; a settling and separation zone is unnecessary since the method also has a physical filtration action; the efficiency of using oxygen is high; and the like, and is popularized.
However, although the biomembrane filtration method as described above is very efficient as to the removal of BOD (biological oxygen demand) and SS (suspended solids) and the nitrification of ammonia, the denitrification which is an anaerobic reaction could not be effectively conducted by this method. For example, in the above described fix bed biomembrane filtration method, it is possible to remove a nitrate nitrogen and a nitrite nitrogen in the water to be treated by stopping the flow of air or oxygen into the packed layer to render the inside of the packed layer anaerobic. However, in this case, a nitrogen gas formed is built up within the packed layer, which prevents formation of a uniform downward flow of the water to be treated, and thus there is a problem of the difficulty of obtaining stabilized removal performance. In addition, in order to remove the nitrate nitrogen and nitrite nitrogen, it is necessary to add a hydrogen donor (for example, an organic substance such as methanol) necessary for the nitrogen removal reaction to the packed layer, and this hydrogen donor sometimes remains in the water to be treated. Accordingly, in order to remove the hydrogen donor such as methanol from the water to be treated, a reaeration tank is typically provided. However, to provide the reaeration tank unfavorably means to add a step and it is clearly preferred to remove the residual hydrogen donor within the biomembrane filtration apparatus.